US authorities last month asked the authors of the research to redact key details in forthcoming publications.

A government advisory panel suggested the data could be used by terrorists.

Biosecurity experts fear an altered, more contagious form of the virus could spark a pandemic deadlier than the 1918-19 Spanish flu outbreak that killed up to 40 million people.

'Right step'

The National Science Advisory Board for Biosecurity (NSABB) recommended key details be omitted from publication of the research, which sparked international furore.

"I would have preferred if this hadn't caused so much controversy, but it has happened and we can't change that," Ron Fouchier, a researcher from Erasmus Medical Center in Rotterdam, told Science Insider.

"So I think it's the right step to make."

While the H5N1 strain of bird flu is extremely deadly when caught by humans, its impact has so far been limited because it is not easily transmissible between humans.

But the latest joint research, by Erasmus University in the Netherlands and the University of Wisconsin-Madison in the US, altered the strain and found it was much more easily passed between ferrets.

Two scientific journals now want to publish the research - albeit in redacted form - and are trying to work out with the US government how to make the data accessible to "responsible scientists".

The NSABB is made up of scientists and public health experts, 23 from outside the government, and 18 from within.

'Flatly false'

It cannot stop publication but makes recommendations to researchers.

The scientists' letter published on Friday argues that knowledge of more infectious strains before they mutate in nature is valuable for public health.

"More research is needed to determine how influenza viruses in nature become human pandemic threats," the statement says, "so that they can be contained before they acquire the ability to transmit from human to human, or so that appropriate countermeasures can be deployed if adaptation to humans occurs."

But some said the 60-day pause on research was not enough.

One critic of the studies, Richard Ebright, a biologist at Rutgers University, told Science Insider that the letter "includes flatly false statements" making assurances about the safety of H1N1 research labs.

Reports say that a meeting debating the research and steps forward could come during a World Health Organization meeting in February.